The immune system, which is made up of special cells, proteins, tissues, and organs, defends people against germs and microorganisms every day. In most cases, the immune system does a great job of keeping people healthy and preventing infections. But sometimes problems with the immune system can lead to illness and infection.

What the Immune System Does

The immune system is the body's defense against infectious organisms and other invaders. Through a series of steps called the immune response, the immune system attacks organisms and substances that invade our systems and cause disease. The immune system is made up of a network of cells, tissues, and organs that work together to protect the body.

The cells that are part of this defense system are white blood cells, or leukocytes. They come in two basic types (more on these below), which combine to seek out and destroy the organisms or substances that cause disease.

Leukocytes are produced or stored in many locations throughout the body, including the thymus, spleen, and bone marrow. For this reason, they are called the lymphoid organs. There are also clumps of lymphoid tissue throughout the body, primarily in the form of lymph nodes, that house the leukocytes.The leukocytes circulate through the body between the organs and nodes by means of the lymphatic vessels. Leukocytes can also circulate through the blood vessels. In this way, the immune system works in a coordinated manner to monitor the body for germs or substances that might cause problems.

The two basic types of leukocytes are:

phagocytes, cells that chew up invading organisms

lymphocytes, cells that allow the body to remember and recognize previous invaders and help the body destroy them

A number of different cells are considered phagocytes. The most common type is the neutrophil, which primarily fights bacteria. If doctors are worried about a bacterial infection, they might order a blood test to see if a patient has an increased number of neutrophils triggered by the infection. Other types of phagocytes have their own jobs to make sure that the body responds appropriately to a specific type of invader.

There are two kinds of lymphocytes: the B lymphocytes and the T lymphocytes. Lymphocytes start out in the bone marrow and either stay there and mature into B cells, or they leave for the thymus gland, where they mature into T cells. B lymphocytes and T lymphocytes have separate jobs to do: B lymphocytes are like the body's military intelligence system, seeking out their targets and sending defenses to lock onto them. T cells are like the soldiers, destroying the invaders that the intelligence system has identified. Here's how it works.

Antigens are foreign substances that invade the body. When an antigen is detected, several types of cells work together to recognize and respond to it. These cells trigger the B lymphocytes to produce antibodies, specialized proteins that lock onto specific antigens. Antibodies and antigens fit together like a key and a lock.

Once the B lymphocytes have produced antibodies, these antibodies continue to exist in a person's body, so that if the same antigen is presented to the immune system again, the antibodies are already there to do their job. That's why if someone gets sick with a certain disease, like chickenpox, that person typically doesn't get sick from it again. This is also why we use immunizations to prevent getting certain diseases. The immunization introduces the body to the antigen in a way that doesn't make a person sick, but it does allow the body to produce antibodies that will then protect that person from future attack by the germ or substance that produces that particular disease.

Although antibodies can recognize an antigen and lock onto it, they are not capable of destroying it without help. That is the job of the T cells. The T cells are part of the system that destroys antigens that have been tagged by antibodies or cells that have been infected or somehow changed. (There are actually T cells that are called "killer cells.") T cells are also involved in helping signal other cells (like phagocytes) to do their jobs.

Antibodies can also neutralize toxins (poisonous or damaging substances) produced by different organisms. Lastly, antibodies can activate a group of proteins called complement that are also part of the immune system. Complement assists in killing bacteria, viruses, or infected cells.

All of these specialized cells and parts of the immune system offer the body protection against disease. This protection is called immunity.

Humans have three types of immunity — innate, adaptive, and passive:

Innate Immunity

Everyone is born with innate (or natural) immunity, a type of general protection that humans have. Many of the germs that affect other species don't harm us. For example, the viruses that cause leukemia in cats or distemper in dogs don't affect humans. Innate immunity works both ways because some viruses that make humans ill — such as the virus that causes HIV/AIDS — don't make cats or dogs sick either.

Innate immunity also includes the external barriers of the body, like the skin and mucous membranes (like those that line the nose, throat, and gastrointestinal tract), which are our first line of defense in preventing diseases from entering the body. If this outer defensive wall is broken (like if you get a cut), the skin attempts to heal the break quickly and special immune cells on the skin attack invading germs.

Adaptive Immunity

We also have a second kind of protection called adaptive (or active) immunity. This type of immunity develops throughout our lives. Adaptive immunity involves the lymphocytes (as in the process described above) and develops as children and adults are exposed to diseases or immunized against diseases through vaccination.

Passive Immunity

Passive immunity is "borrowed" from another source and it lasts for a short time. For example, antibodies in a mother's breast milk provide an infant with temporary immunity to diseases that the mother has been exposed to. This can help protect the infant against infection during the early years of childhood.

Everyone's immune system is different. Some people never seem to get infections, whereas others seem to be sick all the time. As people get older, they usually become immune to more germs as the immune system comes into contact with more and more of them. That's why adults and teens tend to get fewer colds than kids — their bodies have learned to recognize and immediately attack many of the viruses that cause colds.

The circulatory system is composed of the heart and blood vessels, including arteries, veins, and capillaries. Our bodies actually have two circulatory systems: The pulmonary circulation is a short loop from the heart to the lungs and back again, and the systemic circulation (the system we usually think of as our circulatory system) sends blood from the heart to all the other parts of our bodies and back again.

The heart is the key organ in the circulatory system. As a hollow, muscular pump, its main function is to propel blood throughout the body. It usually beats from 60 to 100 times per minute, but can go much faster when necessary. It beats about 100,000 times a day, more than 30 million times per year, and about 2.5 billion times in a 70-year lifetime.

The heart gets messages from the body that tell it when to pump more or less blood depending on an individual's needs. When we're sleeping, it pumps just enough to provide for the lower amounts of oxygen needed by our bodies at rest. When we're exercising or frightened, the heart pumps faster to increase the delivery of oxygen.The heart has four chambers that are enclosed by thick, muscular walls. It lies between the lungs and just to the left of the middle of the chest cavity. The bottom part of the heart is divided into two chambers called the right and left ventricles, which pump blood out of the heart. A wall called the interventricular septum divides the ventricles.

The upper part of the heart is made up of the other two chambers of the heart, the right and left atria. The right and left atria receive the blood entering the heart. A wall called the interatrial septum divides the right and left atria, which are separated from the ventricles by the atrioventricular valves. The tricuspid valve separates the right atrium from the right ventricle, and the mitral valve separates the left atrium and the left ventricle.

Two other cardiac valves separate the ventricles and the large blood vessels that carry blood leaving the heart. These are the pulmonic valve, which separates the right ventricle from the pulmonary artery leading to the lungs, and the aortic valve, which separates the left ventricle from the aorta, the body's largest blood vessel.

Arteries carry blood away from the heart. They are the thickest blood vessels, with muscular walls that contract to keep the blood moving away from the heart and through the body. In the systemic circulation, oxygen-rich blood is pumped from the heart into the aorta. This huge artery curves up and back from the left ventricle, then heads down in front of the spinal column into the abdomen. Two coronary arteries branch off at the beginning of the aorta and divide into a network of smaller arteries that provide oxygen and nourishment to the muscles of the heart.

Unlike the aorta, the body's other main artery, the pulmonary artery, carries oxygen-poor blood. From the right ventricle, the pulmonary artery divides into right and left branches, on the way to the lungs where blood picks up oxygen.

Arterial walls have three layers:

The endothelium is on the inside and provides a smooth lining for blood to flow over as it moves through the artery.

The media is the middle part of the artery, made up of a layer of muscle and elastic tissue.

The adventitia is the tough covering that protects the outside of the artery.

As they get farther from the heart, the arteries branch out into arterioles, which are smaller and less elastic.

Veins carry blood back to the heart. They're not as muscular as arteries, but they contain valves that prevent blood from flowing backward. Veins have the same three layers that arteries do, but are thinner and less flexible. The two largest veins are the superior and inferior vena cavae. The terms superior and inferior don't mean that one vein is better than the other, but that they're located above and below the heart.

A network of tiny capillaries connects the arteries and veins. Though tiny, the capillaries are one of the most important parts of the circulatory system because it's through them that nutrients and oxygen are delivered to the cells. In addition, waste products such as carbon dioxide are also removed by the capillaries.

What the Heart and Circulatory System Do

The circulatory system works closely with other systems in our bodies. It supplies oxygen and nutrients to our bodies by working with the respiratory system. At the same time, the circulatory system helps carry waste and carbon dioxide out of the body.

Hormones — produced by the endocrine system — are also transported through the blood in the circulatory system. As the body's chemical messengers, hormones transfer information and instructions from one set of cells to another. For example, one of the hormones produced by the heart helps control the kidneys' release of salt from the body.

One complete heartbeat makes up a cardiac cycle, which consists of two phases:

In the first phase, the ventricles contract (this is called systole), sending blood into the pulmonary and systemic circulation. To prevent the flow of blood backwards into the atria during systole, the atrioventricular valves close, creating the first sound (the lub). When the ventricles finish contracting, the aortic and pulmonary valves close to prevent blood from flowing back into the ventricles. This is what creates the second sound (the dub).

Then the ventricles relax (this is called diastole) and fill with blood from the atria, which makes up the second phase of the cardiac cycle.

A unique electrical conduction system in the heart causes it to beat in its regular rhythm. The sinoatrial or SA node, a small area of tissue in the wall of the right atrium, sends out an electrical signal to start the contracting of the heart muscle. This node is called the pacemaker of the heart because it sets the rate of the heartbeat and causes the rest of the heart to contract in its rhythm.

These electrical impulses cause the atria to contract first, and then travel down to the atrioventricular or AV node, which acts as a kind of relay station. From here the electrical signal travels through the right and left ventricles, causing them to contract and forcing blood out into the major arteries.

In the systemic circulation, blood travels out of the left ventricle, to the aorta, to every organ and tissue in the body, and then back to the right atrium. The arteries, capillaries, and veins of the systemic circulatory system are the channels through which this long journey takes place.

Once in the arteries, blood flows to smaller arterioles and then to capillaries. While in the capillaries, the bloodstream delivers oxygen and nutrients to the body's cells and picks up waste materials. Blood then goes back through the capillaries into venules, and then to larger veins until it reaches the vena cavae.

Blood from the head and arms returns to the heart through the superior vena cava, and blood from the lower parts of the body returns through the inferior vena cava. Both vena cavae deliver this oxygen-depleted blood into the right atrium. From here the blood exits to fill the right ventricle, ready to be pumped into the pulmonary circulation for more oxygen.

In the pulmonary circulation, blood low in oxygen but high in carbon dioxide is pumped out the right ventricle into the pulmonary artery, which branches off in two directions. The right branch goes to the right lung, and vice versa.

In the lungs, the branches divide further into capillaries. Blood flows more slowly through these tiny vessels, allowing time for gases to be exchanged between the capillary walls and the millions of alveoli, the tiny air sacs in the lungs.

During the process called oxygenation, oxygen is taken up by the bloodstream. Oxygen locks onto a molecule called hemoglobin in the red blood cells. The newly oxygenated blood leaves the lungs through the pulmonary veins and heads back to the heart. It enters the heart in the left atrium, then fills the left ventricle so it can be pumped into the systemic circulation.

We depend on sight more than any other of our senses to maneuver through the space around us. In a single glance, lasting a fraction of a second, our eyes work with our brains to tell us the size, shape, color, and texture of an object. They let us know how close it is, whether it's standing still or coming toward us, and how quickly it's moving. Every day, our eyes give us messages that help us understand the world around us.

Although the eyes are small compared with most of the body's other organs, their structure is incredibly complex. The eyes work together to perceive depth, enabling us to judge distance and the size of objects to help us move around them. Not only do the two eyes work together, they also work with the brain, muscles, and nerves to produce complicated visual images and messages. And our eyes constantly adapt to the changing environment — for example, they are able to adjust so that we can easily move around in a nearly dark room or bright sunlight.

To understand more about how the eyes work, it's important to know about the structures that make up the eye and about conditions and diseases that can interfere with vision.Eyes and What Do They Do

Only part of the eye is visible in a person's face. The whole eye — the eyeball — is about the size and shape of a ping-pong ball.

The eye — both the parts that are visible and those that aren't — is extremely delicate. The body has several ways of protecting this vulnerable organ. The eyeball sits in the eye socket (also called the orbit) in a person's skull, where it is surrounded by bone. The visible part of the eye is protected by the eyelids and the eyelashes, which keep dirt, dust, and even harmful bright light out of the eye.

Our eyes are also protected by tears, which moisten the eyes and clean out dirt, dust, and other irritants that get past the defenses of our eyelashes and eyelids. Tears also help protect the eyes against infection.

Every time we blink, our eyelids spread a layer of mucus, oil, and tears over the cornea, which covers the eye. The lacrimal glands in the upper outer corner of each eye socket produce tears.

After they've done their job moistening the eyes, the tears flow into canals in the eyelids, which drain into the lacrimal sac, a pouch in the lower inner corner of each eye socket. Tears then exit through a passage which leads to the nose.

To see, the eye has to move. Six muscles, called extraocular muscles, surround the eyeball in the skull. These muscles act like the strings on a puppet, moving the eye in different directions. The muscles of each eye normally move together at the same time, allowing the two eyes to remain aligned.

The wall of a person's eyeball is made up of three layers, rather like the layers on an onion:

The sclera is the outermost protective layer. This tough, fibrous tissue surrounds the eyeball and attaches to the cornea, which is the clear front surface of the eye. What we see as the white of the eye is the sclera. Over the sclera lies the conjunctiva, a clear mucous membrane that protects the eye from becoming dry.

The choroid is the middle layer that contains blood vessels that deliver oxygen and nutrients to the retina.

The retina, the innermost of the three layers, lines the inside of the eyeball. The retina is a soft, light-sensitive layer of nervous system tissue. The optic nerve carries signals from the retina to the brain, which interprets them as visual images.

The space in the center of the eyeball is filled with a clear jelly-like material called the vitreous humor. This material allows light to pass through to the retina. It also helps the eye keep its round shape.

Vision is the process by which images captured by the eye are interpreted by the brain, and the visible part of the eye is where the process of sight begins. On the front surface of the eye is the see-through, circle-shaped cornea. You can't see a person's cornea the way you can see the colored part of the eye behind it — the cornea is like a clear window that focuses light into the eye. Behind the cornea is a watery fluid called the aqueous humor. The cornea and aqueous humor form an outer lens that refracts (bends) light on its way into the eye. This is where most of the eye's focusing work is done.

The colored circular membrane in the eye just behind the cornea is called the iris. The iris controls the amount of light entering the eye through the pupil, which is the opening in the center of the iris that looks like a tiny black circle. Like a camera, which controls the amount of light coming in to prevent both overexposure and underexposure, the iris becomes wider and narrower, changing the size of the pupil to control the amount of light entering the eye. The pupil gets bigger when more light is needed to see better and smaller when there's plenty of light.

The eye's lens sits just behind the iris. Just like a camera lens, the eye's lens focuses light to form sharp, clear images. Light that has been focused through the cornea and aqueous humor hits the lens, which then focuses it further, sending the light rays through the vitreous humor and onto the retina.

To focus on objects clearly at varying distances, the eye's lens needs to change shape. The ciliary body contains the muscular structure in the eye that changes the shape of the eye's lens. In people who have normal vision, the ciliary body flattens the lens enough to bring objects into focus at a distance of 20 feet or more. To see closer objects, this muscle contracts to thicken the lens. Young children can see objects at very close range; many people over 45 have to hold objects farther and farther away to see them clearly. This is because the lens becomes less elastic as we age.

The retina (the soft, light-sensitive layer of tissue that lines the back of the eyeball wall) is made up of millions of light receptors. These are called rods and cones. Rods are much more sensitive to light than cones. Each eye has about 125 million rods that help us see in dim light and detect shades of gray, but they cannot distinguish colors. In comparison, the 6 million cones in each eye allow us to see in bright light, and they also sense color and detail.

The macula is a small, specialized area on the retina. The macula helps our eyes see fine details when we look directly at an object. It contains mainly cones and few rods.

When focused light is projected onto the retina, it stimulates the rods and cones. The retina then sends nerve signals are sent through the back of the eye to the optic nerve. The optic nerve carries these signals to the brain, which interprets them as visual images. The portion of the brain that processes visual input and interprets the messages that the eye sends is called the visual cortex.

As in a camera, the eye's lens transmits light patterns upside down. The brain learns that the impulses received from the upper part of the retina are really from the lower part of the object we're seeing and vice versa.

Most people use both eyes to see an object. This is called binocular vision. Through binocular vision, images are formed on the retina of each eye. These images are slightly different, because the object is being viewed from slightly different angles. Nerve signals representing each image are sent to the brain, where they are interpreted as two views of the same object. Some of the nerve fibers from each eye cross, so each side of the brain receives messages from both eyes. Through experience, the brain learns to judge the distance of an object by the degree of difference in the images it receives from the two eyes. This ability to sense distance is called depth perception.

Vision is a fine-tuned process. All the parts of the eye — and the brain — need to work together so a person can see correctly. Because the eye's structure is so complex, though, a lot of things can go wrong.

The foundations of the endocrine system are the hormones and glands. As the body's chemical messengers, hormones transfer information and instructions from one set of cells to another. Although many different hormones circulate throughout the bloodstream, each one affects only the cells that are genetically programmed to receive and respond to its message. Hormone levels can be influenced by factors such as stress, infection, and changes in the balance of fluid and minerals in blood.

A gland is a group of cells that produces and secretes, or gives off, chemicals. A gland selects and removes materials from the blood, processes them, and secretes the finished chemical product for use somewhere in the body. Some types of glands release their secretions in specific areas. For instance, exocrine glands, such as the sweat and salivary glands, release secretions in the skin or inside of the mouth. Endocrine glands, on the other hand, release more than 20 major hormones directly into the bloodstream where they can be transported to cells in other parts of the body.

The major glands that make up the human endocrine system are the hypothalamus, pituitary, thyroid, parathyroids, adrenals, pineal body, and the reproductive glands, which include the ovaries and testes. The pancreas is also part of this hormone-secreting system, even though it is also associated with the digestive system because it also produces and secretes digestive enzymes.Although the endocrine glands are the body's main hormone producers, some non-endocrine organs — such as the brain, heart, lungs, kidneys, liver, thymus, skin, and placenta — also produce and release hormones.

The hypothalamus, a collection of specialized cells that is located in the lower central part of the brain, is the primary link between the endocrine and nervous systems. Nerve cells in the hypothalamus control the pituitary gland by producing chemicals that either stimulate or suppress hormone secretions from the pituitary.

Although it is no bigger than a pea, the pituitary gland, located at the base of the brain just beneath the hypothalamus, is considered the most important part of the endocrine system. It's often called the "master gland" because it makes hormones that control several other endocrine glands. The production and secretion of pituitary hormones can be influenced by factors such as emotions and seasonal changes. To accomplish this, the hypothalamus relays information sensed by the brain (such as environmental temperature, light exposure patterns, and feelings) to the pituitary.

The tiny pituitary is divided into two parts: the anterior lobe and the posterior lobe. The anterior lobe regulates the activity of the thyroid, adrenals, and reproductive glands. Among the hormones it produces are:

growth hormone, which stimulates the growth of bone and other body tissues and plays a role in the body's handling of nutrients and minerals

prolactin, which activates milk production in women who are breastfeeding

thyrotropin, which stimulates the thyroid gland to produce thyroid hormones

corticotropin, which stimulates the adrenal gland to produce certain hormones

The pituitary also secretes endorphins, chemicals that act on the nervous system to reduce sensitivity to pain. In addition, the pituitary secretes hormones that signal the ovaries and testes to make sex hormones. The pituitary gland also controls ovulation and the menstrual cycle in women.

The posterior lobe of the pituitary releases antidiuretic hormone, which helps control body water balance through its effect on the kidneys and urine output; and oxytocin, which triggers the contractions of the uterus that occur during labor.

The thyroid, located in the front part of the lower neck, is shaped like a bowtie or butterfly and produces the thyroid hormones thyroxine and triiodothyronine. These hormones control the rate at which cells burn fuels from food to produce energy. As the level of thyroid hormones increases in the bloodstream, so does the speed at which chemical reactions occur in the body.

Thyroid hormones also play a key role in bone growth and the development of the brain and nervous system in children. The production and release of thyroid hormones is controlled by thyrotropin, which is secreted by the pituitary gland.

Attached to the thyroid are four tiny glands that function together called the parathyroids. They release parathyroid hormone, which regulates the level of calcium in the blood with the help of calcitonin, which is produced in the thyroid.

The body has two triangular adrenal glands, one on top of each kidney. The adrenal glands have two parts, each of which produces a set of hormones and has a different function. The outer part, the adrenal cortex, produces hormones called corticosteroids that influence or regulate salt and water balance in the body, the body's response to stress, metabolism, the immune system, and sexual development and function.

The inner part, the adrenal medulla, produces catecholamines, such as epinephrine. Also called adrenaline, epinephrine increases blood pressure and heart rate when the body experiences stress. (Epinephrine injections are often used to counteract a severe allergic reaction.)

The pineal body, also called the pineal gland, is located in the middle of the brain. It secretes melatonin, a hormone that may help regulate the wake-sleep cycle.

The gonads are the main source of sex hormones. In males, they are located in the scrotum. Male gonads, or testes, secrete hormones called androgens, the most important of which is testosterone. These hormones regulate body changes associated with sexual development, including enlargement of the penis, the growth spurt that occurs during puberty, and the appearance of other male secondary sex characteristics such as deepening of the voice, growth of facial and pubic hair, and the increase in muscle growth and strength. Working with hormones from the pituitary gland, testosterone also supports the production of sperm by the testes.

The female gonads, the ovaries, are located in the pelvis. They produce eggs and secrete the female hormones estrogen and progesterone. Estrogen is involved in the development of female sexual features such as breast growth, the accumulation of body fat around the hips and thighs, and the growth spurt that occurs during puberty. Both estrogen and progesterone are also involved in pregnancy and the regulation of the menstrual cycle.

The pancreas produces (in addition to others) two important hormones, insulin and glucagon. They work together to maintain a steady level of glucose, or sugar, in the blood and to keep the body supplied with fuel to produce and maintain stores of energy.

What the Endocrine System Does

Once a hormone is secreted, it travels from the endocrine gland through the bloodstream to target cells designed to receive its message. Along the way to the target cells, special proteins bind to some of the hormones. The special proteins act as carriers that control the amount of hormone that is available to interact with and affect the target cells.

Also, the target cells have receptors that latch onto only specific hormones, and each hormone has its own receptor, so that each hormone will communicate only with specific target cells that possess receptors for that hormone. When the hormone reaches its target cell, it locks onto the cell's specific receptors and these hormone-receptor combinations transmit chemical instructions to the inner workings of the cell.

When hormone levels reach a certain normal or necessary amount, further secretion is controlled by important body mechanisms to maintain that level of hormone in the blood. This regulation of hormone secretion may involve the hormone itself or another substance in the blood related to the hormone.

For example, if the thyroid gland has secreted adequate amounts of thyroid hormones into the blood, the pituitary gland senses the normal levels of thyroid hormone in the bloodstream and adjusts its release of thyrotropin, the pituitary hormone that stimulates the thyroid gland to produce thyroid hormones.

Another example is parathyroid hormone, which increases the level of calcium in the blood. When the blood calcium level rises, the parathyroid glands sense the change and decrease their secretion of parathyroid hormone. This turnoff process is called a negative feedback system.

What's the first step in the digestive process? Believe it or not, it happens before you even taste your food. Just by smelling that homemade apple pie or thinking about how delicious that ripe tomato is going to be, you start salivating — and the digestive process begins, preparing for that first scrumptious bite.

Food is our fuel, and its nutrients give our bodies' cells the energy and substances they need to operate. But before food can do that, it must be digested into small pieces the body can absorb and use.

About the Digestive System

Almost all animals have a tube-type digestive system in which food enters the mouth, passes through a long tube, and exits as feces (poop) through the anus. The smooth muscle in the walls of the tube-shaped digestive organs rhythmically and efficiently moves the food through the system, where it is broken down into tiny absorbable atoms and molecules.During the process of absorption, nutrients that come from the food (including carbohydrates, proteins, fats, vitamins, and minerals) pass through channels in the intestinal wall and into the bloodstream. The blood works to distribute these nutrients to the rest of the body. The waste parts of food that the body can't use are passed out of the body as feces.

Every morsel of food we eat has to be broken down into nutrients that can be absorbed by the body, which is why it takes hours to fully digest food. In humans, protein must be broken down into amino acids, starches into simple sugars, and fats into fatty acids and glycerol. The water in our food and drink is also absorbed into the bloodstream to provide the body with the fluid it needs.

How Digestion Works

The digestive system is made up of the alimentary canal (also called the digestive tract) and the other abdominal organs that play a part in digestion, such as the liver and pancreas. The alimentary canal is the long tube of organs — including the esophagus, stomach, and intestines — that runs from the mouth to the anus. An adult's digestive tract is about 30 feet (about 9 meters) long.

Digestion begins in the mouth, well before food reaches the stomach. When we see, smell, taste, or even imagine a tasty meal, our salivary glands, which are located under the tongue and near the lower jaw, begin producing saliva. This flow of saliva is set in motion by a brain reflex that's triggered when we sense food or think about eating. In response to this sensory stimulation, the brain sends impulses through the nerves that control the salivary glands, telling them to prepare for a meal.

As the teeth tear and chop the food, saliva moistens it for easy swallowing. A digestive enzyme called amylase, which is found in saliva, starts to break down some of the carbohydrates (starches and sugars) in the food even before it leaves the mouth.

Swallowing, which is accomplished by muscle movements in the tongue and mouth, moves the food into the throat, or pharynx. The pharynx, a passageway for food and air, is about 5 inches (12.7 centimeters) long. A flexible flap of tissue called the epiglottis reflexively closes over the windpipe when we swallow to prevent choking.

From the throat, food travels down a muscular tube in the chest called the esophagus. Waves of muscle contractions called peristalsis force food down through the esophagus to the stomach. A person normally isn't aware of the movements of the esophagus, stomach, and intestine that take place as food passes through the digestive tract.

At the end of the esophagus, a muscular ring or valve called a sphincter allows food to enter the stomach and then squeezes shut to keep food or fluid from flowing back up into the esophagus. The stomach muscles churn and mix the food with acids and enzymes, breaking it into much smaller, digestible pieces. An acidic environment is needed for the digestion that takes place in the stomach. Glands in the stomach lining produce about 3 quarts (2.8 liters) of these digestive juices each day.

Most substances in the food we eat need further digestion and must travel into the intestine before being absorbed. When it's empty, an adult's stomach has a volume of one fifth of a cup (1.6 fluid ounces), but it can expand to hold more than 8 cups (64 fluid ounces) of food after a large meal.

By the time food is ready to leave the stomach, it has been processed into a thick liquid called chyme. A walnut-sized muscular valve at the outlet of the stomach called the pylorus keeps chyme in the stomach until it reaches the right consistency to pass into the small intestine. Chyme is then squirted down into the small intestine, where digestion of food continues so the body can absorb the nutrients into the bloodstream.

The small intestine is made up of three parts:

the duodenum, the C-shaped first part

the jejunum, the coiled midsection

the ileum, the final section that leads into the large intestine

The inner wall of the small intestine is covered with millions of microscopic, finger-like projections called villi. The villi are the vehicles through which nutrients can be absorbed into the body.

The liver (located under the rib cage in the right upper part of the abdomen), the gallbladder (hidden just below the liver), and the pancreas (beneath the stomach) are not part of the alimentary canal, but these organs are essential to digestion.

The liver produces bile, which helps the body absorb fat. Bile is stored in the gallbladder until it is needed. The pancreas produces enzymes that help digest proteins, fats, and carbohydrates. It also makes a substance that neutralizes stomach acid. These enzymes and bile travel through special channels (called ducts) directly into the small intestine, where they help to break down food. The liver also plays a major role in the handling and processing of nutrients, which are carried to the liver in the blood from the small intestine.

From the small intestine, undigested food (and some water) travels to the large intestine through a muscular ring or valve that prevents food from returning to the small intestine. By the time food reaches the large intestine, the work of absorbing nutrients is nearly finished. The large intestine's main function is to remove water from the undigested matter and form solid waste that can be excreted. The large intestine is made up of these three parts:

The cecum is a pouch at the beginning of the large intestine that joins the small intestine to the large intestine. This transition area expands in diameter, allowing food to travel from the small intestine to the large. The appendix, a small, hollow, finger-like pouch, hangs at the end of the cecum. Doctors believe the appendix is left over from a previous time in human evolution. It no longer appears to be useful to the digestive process.

The colon extends from the cecum up the right side of the abdomen, across the upper abdomen, and then down the left side of the abdomen, finally connecting to the rectum. The colon has three parts: the ascending colon; the transverse colon, which absorb fluids and salts; and the descending colon, which holds the resulting waste. Bacteria in the colon help to digest the remaining food products.

The rectum is where feces are stored until they leave the digestive system through the anus as a bowel movement.

If you think of the brain as a central computer that controls all bodily functions, then the nervous system is like a network that relays messages back and forth from the brain to different parts of the body. It does this via the spinal cord, which runs from the brain down through the back and contains threadlike nerves that branch out to every organ and body part.

When a message comes into the brain from anywhere in the body, the brain tells the body how to react. For example, if you accidentally touch a hot stove, the nerves in your skin shoot a message of pain to your brain. The brain then sends a message back telling the muscles in your hand to pull away. Luckily, this neurological relay race takes a lot less time than it just took to read about it.Considering everything it does, the human brain is incredibly compact, weighing just 3 pounds. Its many folds and grooves, though, provide it with the additional surface area necessary for storing all of the body's important information.

The spinal cord, on the other hand, is a long bundle of nerve tissue about 18 inches long and ¾ inch thick. It extends from the lower part of the brain down through spine. Along the way, various nerves branch out to the entire body. These are called the peripheral nervous system.

Both the brain and the spinal cord are protected by bone: the brain by the bones of the skull, and the spinal cord by a set of ring-shaped bones called vertebrae. They're both cushioned by layers of membranes called meninges as well as a special fluid called cerebrospinal fluid. This fluid helps protect the nerve tissue, keep it healthy, and remove waste products.

The brain is made up of three main sections: the forebrain, the midbrain, and the hindbrain.

The ForebrainThe forebrain is the largest and most complex part of the brain. It consists of the cerebrum — the area with all the folds and grooves typically seen in pictures of the brain — as well as some other structures beneath it.

The cerebrum contains the information that essentially makes us who we are: our intelligence, memory, personality, emotion, speech, and ability to feel and move. Specific areas of the cerebrum are in charge of processing these different types of information. These are called lobes, and there are four of them: the frontal, parietal, temporal, and occipital.

The cerebrum has right and left halves, called hemispheres, which are connected in the middle by a band of nerve fibers (the corpus collosum) that enables the two sides to communicate. Though these halves may look like mirror images of each other, many scientists believe they have different functions. The left side is considered the logical, analytical, objective side. The right side is thought to be more intuitive, creative, and subjective. So when you're balancing the checkbook, you're using the left side; when you're listening to music, you're using the right side. It's believed that some people are more "right-brained" or "left-brained" while others are more "whole-brained," meaning they use both halves of their brain to the same degree.

The outer layer of the cerebrum is called the cortex (also known as "gray matter"). Information collected by the five senses comes into the brain from the spinal cord to the cortex. This information is then directed to other parts of the nervous system for further processing. For example, when you touch the hot stove, not only does a message go out to move your hand but one also goes to another part of the brain to help you remember not to do that again.

In the inner part of the forebrain sits the thalamus, hypothalamus, and pituitary gland. The thalamus carries messages from the sensory organs like the eyes, ears, nose, and fingers to the cortex. The hypothalamus controls the pulse, thirst, appetite, sleep patterns, and other processes in our bodies that happen automatically. It also controls the pituitary gland, which makes the hormones that control our growth, metabolism, digestion, sexual maturity, and response to stress.

The MidbrainThe midbrain, located underneath the middle of the forebrain, acts as a master coordinator for all the messages going in and out of the brain to the spinal cord.

The HindbrainThe hindbrain sits underneath the back end of the cerebrum, and it consists of the cerebellum, pons, and medulla. The cerebellum — also called the "little brain" because it looks like a small version of the cerebrum — is responsible for balance, movement, and coordination.

The pons and the medulla, along with the midbrain, are often called the brainstem. The brainstem takes in, sends out, and coordinates all of the brain's messages. It is also controls many of the body's automatic functions, like breathing, heart rate, blood pressure, swallowing, digestion, and blinking.

How the Nervous System Works

The basic functioning of the nervous system depends a lot on tiny cells called neurons. The brain has billions of them, and they have many specialized jobs. For example, sensory neurons take information from the eyes, ears, nose, tongue, and skin to the brain. Motor neurons carry messages away from the brain and back to the rest of the body.

All neurons, however, relay information to each other through a complex electrochemical process, making connections that affect the way we think, learn, move, and behave.

Intelligence, learning, and memory. At birth, the nervous system contains all the neurons you will ever have, but many of them are not connected to each other. As you grow and learn, messages travel from one neuron to another over and over, creating connections, or pathways, in the brain. It's why driving seemed to take so much concentration when you first learned but now is second nature: The pathway became established.

In young children, the brain is highly adaptable; in fact, when one part of a young child's brain is injured, another part can often learn to take over some of the lost function. But as we age, the brain has to work harder to make new neural pathways, making it more difficult to master new tasks or change established behavior patterns. That's why many scientists believe it's important to keep challenging your brain to learn new things and make new connections— it helps keeps the brain active over the course of a lifetime.

Memory is another complex function of the brain. The things we've done, learned, and seen are first processed in the cortex, and then, if we sense that this information is important enough to remember permanently, it's passed inward to other regions of the brain (such as the hippocampus and amygdala) for long-term storage and retrieval. As these messages travel through the brain, they too create pathways that serve as the basis of our memory.

Movement. Different parts of the cerebrum are responsible for moving different body parts. The left side of the brain controls the movements of the right side of the body, and the right side of the brain controls the movements of the left side of the body. When you press the accelerator with your right foot, for example, it's the left side of your brain that sends the message allowing you to do it.

Basic body functions. A part of the peripheral nervous system called the autonomic nervous system is responsible for controlling many of the body processes we almost never need to think about, like breathing, digestion, sweating, and shivering. The autonomic nervous system has two parts: the sympathetic and the parasympathetic nervous systems.

The sympathetic nervous system prepares the body for sudden stress, like if you see a robbery taking place. When something frightening happens, the sympathetic nervous system makes the heart beat faster so that it sends blood more quickly to the different body parts that might need it. It also causes the adrenal glands at the top of the kidneys to release adrenaline, a hormone that helps give extra power to the muscles for a quick getaway. This process is known as the body's "fight or flight" response.

The parasympathetic nervous system does the exact opposite: It prepares the body for rest. It also helps the digestive tract move along so our bodies can efficiently take in nutrients from the food we eat.

The senses. Your spouse may be a sight for sore eyes at the end of a long day — but without the brain, you wouldn't even recognize him or her. Pepperoni pizza sure is delicious — but without the brain, your taste buds wouldn't be able to tell if you were eating pizza or the box it came in. None of your senses would be useful without the processing that occurs in the brain.

Sight. Sight probably tells us more about the world than any other sense. Light entering the eye forms an upside-down image on the retina. The retina transforms the light into nerve signals for the brain. The brain then turns the image right-side up and tells us what we are seeing.

Hearing. Every sound we hear is the result of sound waves entering our ears and causing our eardrums to vibrate. These vibrations are then transferred along the tiny bones of the middle ear and converted into nerve signals. The cortex then processes these signals, telling us what we are hearing.

Taste. The tongue contains small groups of sensory cells called taste buds that react to chemicals in foods. Taste buds react to sweet, sour, salty, and bitter. Messages are sent from the taste buds to the areas in the cortex responsible for processing taste.

Smell. Olfactory cells in the mucous membranes lining each nostril react to chemicals we breathe in and send messages along specific nerves to the brain— which, according to experts, can distinguish between more than 10,000 different smells. With that kind of sensitivity, it's no wonder research suggests that smells are very closely linked to our memories.

Touch. The skin contains more than 4 million sensory receptors — mostly concentrated in the fingers, tongue, and lips — that gather information related to touch, pressure, temperature, and pain and send it to the brain for processing and reaction.

Joints occur where two bones meet. They make the skeleton flexible — without them, movement would be impossible.

Joints allow our bodies to move in many ways. Some joints open and close like a hinge (such as knees and elbows), whereas others allow for more complicated movement — a shoulder or hip joint, for example, allows for backward, forward, sideways, and rotating movement.

Joints are classified by their range of movement. Immovable, or fibrous, joints don't move. The dome of the skull, for example, is made of bony plates, which must be immovable to protect the brain. Between the edges of these plates are links, or joints, of fibrous tissue. Fibrous joints also hold the teeth in the jawbone.Partially movable, or cartilaginous, joints move a little. They are linked by cartilage, as in the spine. Each of the vertebrae in the spine moves in relation to the one above and below it, and together these movements give the spine its flexibility.

Freely movable, or synovial, joints move in many directions. The main joints of the body — found at the hip, shoulders, elbows, knees, wrists, and ankles — are freely movable. They are filled with synovial fluid, which acts as a lubricant to help the joints move easily.

Three kinds of freely movable joints play a big part in voluntary movement:

Hinge joints allow movement in one direction, as seen in the knees and elbows.

Pivot joints allow a rotating or twisting motion, like that of the head moving from side to side.

Ball-and-socket joints allow the greatest freedom of movement. The hips and shoulders have this type of joint, in which the round end of a long bone fits into the hollow of another bone.

Problems With the Bones, Muscles, and Joints

As strong as bones are, they can break. Muscles can weaken, and joints (as well as tendons, ligaments, and cartilage) can be damaged by injury or disease.

Problems that can affect the bones, muscles, and joints include:

Arthritis. Arthritis is the inflammation of a joint, and people who have it experience swelling, warmth, pain, and often have trouble moving. Although we often think of arthritis as a condition that affects only older people, arthritis can also occur in children and teens. Health problems that involve arthritis in kids and teens include juvenile rheumatoid arthritis (JRA), lupus, Lyme disease, and septic arthritis (a bacterial infection of a joint).

Fracture. A fracture occurs when a bone breaks; it may crack, snap, or shatter. After a fracture, new bone cells fill the gap and repair the break. Applying a strong plaster cast, which keeps the bone in the correct position until it heals, is the usual treatment. If the fracture is complicated, metal pins and plates can be placed to better stabilize it while the bone heals.

Muscular dystrophy. Muscular dystrophy is an inherited group of diseases that affect the muscles, causing them to weaken and break down over time. The most common form in childhood is called Duchenne muscular dystrophy, and it most often affects boys.

Osgood-Schlatter disease(OSD). Osgood-Schlatter disease is an inflammation (pain and swelling) of the bone, cartilage, and/or tendon at the top of the shinbone, where the tendon from the kneecap attaches. OSD usually strikes active teens around the beginning of their growth spurts, the approximately 2-year period during which they grow most rapidly.

Osteomyelitis. Osteomyelitis is a bone infection often caused by Staphylococcus aureus bacteria, though other types of bacteria can cause it, too. In kids and teens, osteomyelitis usually affects the long bones of the arms and legs. Osteomyelitis often develops after an injury or trauma.

Osteoporosis. In osteoporosis, bone tissue becomes brittle, thin, and spongy. Bones break easily, and the spine sometimes begins to crumble and collapse. Although the condition usually affects older people, kids and teens with eating disorders can get the condition, as can girls with female athlete triad — a combination of three conditions that some girls who exercise or play sports may be at risk for: disordered eating, amenorrhea (loss of a girl's period), and osteoporosis. Participation in sports where a thin appearance is valued can put a girl at risk for female athlete triad.

Repetitive stress injuries (RSIs). RSIs are a group of injuries that happen when too much stress is placed on a part of the body, resulting in inflammation (pain and swelling), muscle strain, or tissue damage. This stress generally occurs from repeating the same movements over and over again. RSIs are becoming more common in kids and teens because they spend more time than ever using computers. Playing sports like tennis that involve repetitive motions can also lead to RSIs. Kids and teens who spend a lot of time playing musical instruments or video games are also at risk for RSIs.

Scoliosis. Every person's spine curves a little bit; a certain amount of curvature is necessary for people to move and walk properly. But 3–5 people out of 1,000 have scoliosis, which causes the spine to curve too much. It can be hereditary, so someone who has scoliosis often has family members who have it.

Strains and sprains. Strains occur when muscles or tendons are overstretched. Sprains are an overstretching or a partial tearing of the ligaments. Strains usually happen when a person takes part in a strenuous activity when the muscles haven't properly warmed up or the muscle is not used to the activity (such as a new sport or playing a familiar sport after a long break). Sprains, on the other hand, are usually the result of an injury, such as twisting an ankle or knee. A common sprain injury is a torn Achilles tendon, which connects the calf muscles to the heel. This tendon can snap, but it usually can be repaired by surgery. Both strains and sprains are common in kids and teens because they're active and still growing.

Tendinitis. This common sports injury that usually happens after overexercising a muscle. The tendon and tendon sheath become inflamed, which can be painful. Resting the muscles and taking anti-inflammatory medication can bring relief.

Bones don't work alone — they need help from the muscles and joints. Muscles pull on the joints, allowing us to move. They also help your body perform other functions so you can grow and remain strong, such as chewing food and then moving it through the digestive system.

The human body has more than 650 muscles, which make up half of a person's body weight. They are connected to bones by tough, cord-like tissues called tendons, which allow the muscles to pull on bones. If you wiggle your fingers, you can see the tendons on the back of your hand move as they do their work.Humans have three different kinds of muscle:

Skeletal muscle is attached to bone, mostly in the legs, arms, abdomen, chest, neck, and face. Skeletal muscles are called striated because they are made up of fibers that have horizontal stripes when viewed under a microscope. These muscles hold the skeleton together, give the body shape, and help it with everyday movements (known as voluntary muscles because you can control their movement). They can contract (shorten or tighten) quickly and powerfully, but they tire easily and have to rest between workouts.

Smooth, or involuntary, muscle is also made of fibers, but this type of muscle looks smooth, not striated. Generally, we can't consciously control our smooth muscles; rather, they're controlled by the nervous system automatically (which is why they're also called involuntary). Examples of smooth muscles are the walls of the stomach and intestines, which help break up food and move it through the digestive system. Smooth muscle is also found in the walls of blood vessels, where it squeezes the stream of blood flowing through the vessels to help maintain blood pressure. Smooth muscles take longer to contract than skeletal muscles do, but they can stay contracted for a long time because they don't tire easily.

Cardiac muscle is found in the heart. The walls of the heart's chambers are composed almost entirely of muscle fibers. Cardiac muscle is also an involuntary type of muscle. Its rhythmic, powerful contractions force blood out of the heart as it beats.

Even when you sit perfectly still, muscles throughout your body are constantly moving. Muscles enable your heart to beat, your chest to rise and fall as you breathe, and your blood vessels to help regulate the pressure and flow of blood through your body. When we smile and talk, muscles are helping us communicate, and when we exercise, they help us stay physically fit and healthy.

The movements your muscles make are coordinated and controlled by the brain and nervous system. The involuntary muscles are controlled by structures deep within the brain and the upper part of the spinal cord called the brain stem. The voluntary muscles are regulated by the parts of the brain known as the cerebral motor cortex and the cerebellum.

When you decide to move, the motor cortex sends an electrical signal through the spinal cord and peripheral nerves to the muscles, causing them to contract. The motor cortex on the right side of the brain controls the muscles on the left side of the body and vice versa.

The cerebellum coordinates the muscle movements ordered by the motor cortex. Sensors in the muscles and joints send messages back through peripheral nerves to tell the cerebellum and other parts of the brain where and how the arm or leg is moving and what position it's in. This feedback results in smooth, coordinated motion. If you want to lift your arm, your brain sends a message to the muscles in your arm and you move it. When you run, the messages to the brain are more involved, because many muscles have to work in rhythm.

Muscles move body parts by contracting and then relaxing. Your muscles can pull bones, but they can't push them back to the original position. So they work in pairs of flexors and extensors. The flexor contracts to bend a limb at a joint. Then, when you've completed the movement, the flexor relaxes and the extensor contracts to extend or straighten the limb at the same joint. For example, the biceps muscle, in the front of the upper arm, is a flexor, and the triceps, at the back of the upper arm, is an extensor. When you bend at your elbow, the biceps contracts. Then the biceps relaxes and the triceps contracts to straighten the elbow.

From our head to our toes, our bones provide support for our bodies and help form our shape. The skull protects the brain and forms the shape of our face. The spinal cord, a pathway for messages between the brain and the body, is protected by the backbone, or spinal column.

The ribs form a cage that shelters the heart, lungs, liver, and spleen, and the pelvis helps protect the bladder, intestines, and in women, the reproductive organs.

Although they're very light, bones are strong enough to support our entire weight.

The human skeleton has 206 bones, which begin to develop before birth. When the skeleton first forms, it is made of flexible cartilage, but within a few weeks it begins the process of ossification. Ossification is when the cartilage is replaced by hard deposits of calcium phosphate and stretchy collagen, the two main components of bone. It takes about 20 years for this process to be completed.

The bones of kids and young teens are smaller than those of adults and contain "growing zones" called growth plates. These plates consist of columns of multiplying cartilage cells that grow in length, and then change into hard, mineralized bone. These growth plates are easy to spot on an X-ray. Because girls mature at an earlier age than boys, their growth plates change into hard bone at an earlier age.

Bone building continues throughout life, as a body constantly renews and reshapes the bones' living tissue. Bone contains three types of cells: osteoblasts, which make new bone and help repair damage; osteocytes, which carry nutrients and waste products to and from blood vessels in the bone; and osteoclasts, which break down bone and help to sculpt and shape it.

Osteoclasts are very active in kids and teens, working on bone as it is remodeled during growth. They also play an important role in the repair of fractures.

Bones are made up of calcium, phosphorus, sodium, and other minerals, as well as the protein collagen. Calcium is needed to make bones hard, which allows them to support body weight. Bones also store calcium and release some into the bloodstream when it's needed by other parts of the body. The amounts of certain vitamins and minerals that you eat, especially vitamin D and calcium, directly affects how much calcium is stored in the bones.

The soft bone marrow inside many of the bones is where most of the blood cells are made. The bone marrow contains stem cells, which produce the body's red blood cells and platelets. Red blood cells carry oxygen to the body's tissues, and platelets help with blood clotting when someone has a cut or wound.

Bones are made up of two types of bone:

Compact bone is the solid, hard, outside part of the bone. It looks like ivory and is extremely strong. Holes and channels run through it, carrying blood vessels and nerves from the periosteum, the bone's membrane covering, to its inner parts.

Cancellous bone, which looks like a sponge, is inside the compact bone. It is made up of a mesh-like network of tiny pieces of bone called trabeculae. The spaces in this network are filled with red marrow, found mainly at the ends of bones, and yellow marrow, which is mostly fat.

Bones are fastened to other bones by long, fibrous straps called ligaments. Cartilage, a flexible, rubbery substance in our joints, supports bones and protects them where they rub against each other.

Humans can't live without blood. Without blood, the body's organs couldn't get the oxygen and nutrients they need to survive, we couldn't keep warm or cool off, fight infections, or get rid of our own waste products. Without enough blood, we'd weaken and die.

Here are the basics about the mysterious, life-sustaining fluid called blood.

Blood and What It Does

Two types of blood vessels carry blood throughout our bodies: The arteries carry oxygenated blood (blood that has received oxygen from the lungs) from the heart to the rest of the body. The blood then travels through the veins back to the heart and lungs, where it receives more oxygen.

As the heart beats, you can feel blood traveling through the body at pulse points — like the neck and the wrist — where large, blood-filled arteries run close to the surface of the skin.

The blood that flows through this network of veins and arteries is called whole blood, and it contains three types of blood cells:

blood cells (RBCs)

white blood cells (WBCs)

platelets

These blood cells are mostly manufactured in the bone marrow (the soft tissue inside our bones), especially in the bone marrow of the vertebrae (the bones that make up the spine), ribs, pelvis, skull, and sternum (breastbone).

The cells travel through the circulatory system suspended in a yellowish fluid called plasma. Plasma is 90% water and contains nutrients, proteins, hormones, and waste products. Whole blood is a mixture of blood cells and plasma.

Red blood cells (also called erythrocytes) are shaped like slightly indented, flattened disks. RBCs contain the iron-rich protein hemoglobin. Blood gets its bright red color when hemoglobin picks up oxygen in the lungs. As the blood travels through the body, the hemoglobin releases oxygen to the tissues. The body contains more RBCs than any other type of cell, and each has a life span of about 4 months. Each day, the body produces new red blood cells to replace those that die or are lost from the body.

White blood cells (also called leukocytes) are a key part of the body's system for defending itself against infection. They can move in and out of the bloodstream to reach affected tissues. The blood contains far fewer WBCs than red cells, although the body can increase production of WBCs to fight infection. There are several types of WBCs, and their life spans vary from a few days to months. New cells are constantly being formed in the bone marrow.

Several different parts of blood are involved in fighting infection. White blood cells called granulocytes and lymphocytes travel along the walls of blood vessels. They fight germs such as bacteria and viruses and may also attempt to destroy cells that have become infected or have changed into cancer cells.

Certain types of WBCs produce antibodies, special proteins that recognize foreign materials and help the body destroy or neutralize them. The white cell count (the number of cells in a given amount of blood) in someone with an infection often is higher than usual because more WBCs are being produced or are entering the bloodstream to battle the infection. After the body has been challenged by some infections, lymphocytes "remember" how to make the specific antibodies that will quickly attack the same germ if it enters the body again.

Platelets (also called thrombocytes) are tiny oval-shaped cells made in the bone marrow. They help in the clotting process. When a blood vessel breaks, platelets gather in the area and help seal off the leak. Platelets survive only about 9 days in the bloodstream and are constantly being replaced by new cells.

Important proteins called clotting factors are critical to the clotting process. Although platelets alone can plug small blood vessel leaks and temporarily stop or slow bleeding, the action of clotting factors is needed to produce a strong, stable clot.

Platelets and clotting factors work together to form solid lumps to seal leaks, wounds, cuts, and scratches and to prevent bleeding inside and on the surfaces of our bodies. The process of clotting is like a puzzle with interlocking parts. When the last part is in place, the clot happens — but if even one piece is missing, the final pieces can't come together.

When large blood vessels are severed (or cut), the body may not be able to repair itself through clotting alone. In these cases, dressings or stitches are used to help control bleeding.

Blood contains other important substances, such as nutrients from food that has been processed by the digestive system. Blood also carries hormones released by the endocrine glands and carries them to the body parts that need them.

Blood is essential for good health because the body depends on a steady supply of fuel and oxygen to reach its billions of cells. Even the heart couldn't survive without blood flowing through the vessels that bring nourishment to its muscular walls.

Blood also carries carbon dioxide and other waste materials to the lungs, kidneys, and digestive system to be removed from the body.

Blood cells and some of the special proteins blood contains can be replaced or supplemented by giving a person blood from someone else via a transfusion. In addition to receiving whole-blood transfusions, people can also receive transfusions of a particular component of blood, such as platelets, RBCs, or a clotting factor. When someone donates blood, the whole blood can be separated into its different parts to be used in this way.Things That Can Go Wrong With Blood

Most of the time, blood functions without problems, but sometimes, blood disorders or diseases can cause illness. Diseases of the blood that commonly affect kids can involve any or all of the three types of blood cells. Other types of blood diseases affect the proteins and chemicals in the plasma that are responsible for clotting.Diseases of the Red Blood Cells

The most common condition affecting RBCs is anemia, a lower-than-normal number of red cells in the blood. Anemia is accompanied by a decrease in the amount of hemoglobin. The symptoms of anemia — such as pale skin, weakness, a fast heart rate, and poor growth in infants and children — happen because of the blood's reduced capacity for carrying oxygen.

Anemia typically is caused by either inadequate RBC production or unusually rapid RBC destruction. In severe cases of chronic anemia, or when a large amount of blood is lost, someone may need a transfusion of RBCs or whole blood.

Anemia resulting from inadequate RBC production. Conditions that can cause a reduced production of red blood cells include:

Iron deficiency anemia. The most common type of anemia, it affects kids and teens of any age who have a diet low in iron or who've lost a lot of RBCs (and the iron they contain) through bleeding. Premature babies, infants with poor nutrition, menstruating teenage girls, and those with ongoing blood loss due to illnesses such as inflammatory bowel disease are especially likely to have iron deficiency anemia.

Lead poisoning. When lead enters the body, most of it goes into RBCs where it can interfere with the production of hemoglobin. This can result in anemia. Lead poisoning can also affect — and sometimes permanently damage — other body tissues, including the brain and nervous system. Although lead poisoning is much less common now, it still is a problem in many larger cities, especially where young children might ingest paint chips or the dust that comes from lead-containing paints peeling off the walls in older buildings.

Anemia due to chronic disease. Kids with chronic diseases (such as cancer or human immunodeficiency virus infection) often develop anemia as a complication of their illness.

Anemia due to kidney disease. The kidneys produce erythropoietin, a hormone that stimulates production of red cells in the bone marrow. Kidney disease can interfere with the production of this hormone.

Anemia resulting from unusually rapid red blood cell destruction. When RBCs are destroyed more quickly than normal by disease (a process called hemolysis), the bone marrow will make up for it by increasing production of new red cells to take their place. But if RBCs are destroyed faster than they can be replaced, a person will develop anemia.

G6PD deficiency. G6PD is an enzyme that helps to protect red blood cells from the destructive effects of certain chemicals found in foods and medications. When the enzyme is deficient, these chemicals can cause red cells to hemolyze, or burst. G6PD deficiency is a common hereditary disease among people of African, Mediterranean, and Southeast Asian descent.

Hereditary spherocytosis is an inherited condition in which RBCs are misshapen (like tiny spheres, instead of disks) and especially fragile because of a genetic problem with a protein in the structure of the red blood cell. This fragility causes the cells to be easily destroyed.

Autoimmune hemolytic anemia. Sometimes — because of disease or for no known reason — the body's immune system mistakenly attacks and destroys RBCs.

Sickle cell anemia, most common in people of African descent, is a hereditary disease that results in the production of abnormal hemoglobin. The RBCs become sickle shaped, they cannot carry oxygen adequately, and they are easily destroyed. The sickle-shaped blood cells also tend to abnormally stick together, causing obstruction of blood vessels. This blockage in the blood vessels can seriously damage organs and cause bouts of severe pain.

Diseases of the White Blood Cells

Neutropenia occurs when there aren't enough of a certain type of white blood cell to protect the body against bacterial infections. People who take certain chemotherapy drugs to treat cancer may develop neutropenia.

Human immunodeficiency virus (HIV) is a virus that attacks certain types of WBCs (lymphocytes) that work to fight infection. Infection with the virus can result in AIDS (acquired immunodeficiency syndrome), leaving the body prone to infections and certain other diseases. Newborns can become infected with the virus from their infected mothers while in the uterus, during birth, or from breastfeeding, although HIV infection of the fetus and newborn is usually preventable with proper medical treatment of the mother during pregnancy and delivery. Teens and adults can get HIV from sex with an infected person or from sharing contaminated needles used for injecting drugs or tattoo ink.

Leukemias are cancers of the cells that produce WBCs. These cancers include acute myeloid leukemia (AML), chronic myeloid leukemia (CML), acute lymphocytic leukemia (ALL), and chronic lymphocytic leukemia (CLL). The most common types of leukemia affecting kids are ALL and AML. In the past 25 years, scientists have made great advances in treating several types of childhood leukemia, most notably certain types of ALL.

Diseases of Platelets

Thrombocytopenia, or a lower than normal number of platelets, is usually diagnosed because a person has abnormal bruising or bleeding. Thrombocytopenia can happen when someone takes certain drugs or develops infections or leukemia or when the body uses up too many platelets. Idiopathic thrombocytopenic purpura (ITP) is a condition in which the immune system attacks and destroys platelets.

Diseases of the Clotting System

The body's clotting system depends on platelets as well as many clotting factors and other blood components. If a hereditary defect affects any of these components, a person can have a bleeding disorder.

Common bleeding disorders include:

Hemophilia, an inherited condition that almost exclusively affects boys, involves a lack of particular clotting factors in the blood. People with severe hemophilia are at risk for excessive bleeding and bruising after dental work, surgery, and trauma. They may experience episodes of life-threatening internal bleeding, even if they haven't been injured.

Von Willebrand disease, the most common hereditary bleeding disorder, also involves a clotting-factor deficiency. It affects both males and females.

Other causes of clotting problems include chronic liver disease (clotting factors are produced in the liver) and vitamin K deficiency (the vitamin is necessary for the production of certain clotting factors).

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